The present invention generally relates to semiconductor devices and more particularly to a a planar laser diode having a reduced optical loss.
Planar laser diode is an essential device in the field of optical parallel processing including optical computing and optical interconnection. In the planar laser diodes, an optical beam is emitted in the direction perpendicular to the plane of the substrate. Thus, the planar laser diode has an optical cavity that causes a resonance in the vertical direction to the plane of the substrate. Such a planar laser diode is particularly advantageous with regard to the design of optical computing system as well as with regard to the possibility of expansion of the system. Further, the planar laser diode has various advantageous features such as low threshold current of oscillation associated with the small volume of optical cavity, single mode operation caused by the small optical cavity length, easy coupling to an optical fiber due to the small beam divergence, high yield of production associated with the elimination of cleavage process in the fabrication process, and easy test of the device in the state that the laser diodes are formed on the wafer.
FIG. 1 shows the construction of a conventional planar laser diode.
Referring to FIG. 1, the laser diode is constructed on a semiconductor substrate 21 of a first conductivity type, and a first reflecting structure 22 is provided on the substrate 21, wherein the reflecting structure 22 is formed of a number of semiconductor layers stacked with each other. More particularly, the first reflecting structure 22 is formed of an alternate stacking of a first semiconductor layer having a first refractive index and a second semiconductor layer having a second, different refractive index, wherein the first and second semiconductor layers forming the first reflecting structure 22 are doped to the first conductivity type. Further, a cavity structure 23 is provided on the first reflecting structure 22 with a thickness set equal to an integer multiple of one-half the wavelength of the optical beam to be produced by the laser diode. The cavity structure includes an active layer 23a sandwiched from above and below with a pair of cladding layers 23b, and produces an optical beam by stimulated emission. Further, a second reflecting structure 24, which has a similar construction as the first reflecting structure 22 except that the semiconductor layers forming the reflecting structure 24 are doped to a second, opposite conductivity type, is provided on the cavity structure 23. Further, a transparent electrode 25 is provided on the lower major surface of the substrate 21, and an electrode 26 is provided on the upper major surface of the second reflecting structure 24. In the structure of FIG. 1, the first and second reflecting structures 22 and 24 are formed such that the reflectivity exceeding 99% is obtained.
In the structure of FIG. 1, it should be noted that the injection of the current into the active layer 23a in the cavity structure 23 is achieved via the first and second reflecting structures 22 and 24. Thus, in order to achieve an efficient injection of the current, it is necessary to set the resistivity of the reflecting structures 22 and 24 and hence the resistivity of the semiconductor layers forming the reflecting structures 22 and 23 as small as possible. In order to reduce the resistivity of a semiconductor layer, it is necessary to increase the carrier density by increasing the impurity concentration level in the reflecting structures 22 and 23. On the other hand, such an increased carrier density in turn invites an increased absorption of the optical beam by the free carrier absorption in which carriers are excited upon absorption of optical radiation. It should be noted that such an absorption caused by the free carrier absorption appears conspicuously with increasing number of the semiconductor layers forming the reflecting structures 22 and 24.
FIG. 2 shows the relationship between the number of stacks of a GaAs layer and an AlAs layer that form together the reflecting structure 22 or 24 and the reflectivity, wherein the vertical axis represents the reflectivity and the horizontal axis represents the number of the stacks in a logarithmic scale. In view of the fact that the absorption of a semiconductor layer increases with increasing level of doping, it is expected that the tendency of saturation of reflectivity in FIG. 2 is even more enhanced when the impurity concentration level is increased in the reflecting structures 22 and 24. Further, it should be noted that resistivity of the reflecting structures 22 and 24 would increase with increasing number of the stacks, as each stack of the GaAs layer and the AlAs layer includes a heterojunction interface and associated potential barrier of carriers. Obviously, such potential barriers act to prevent the passage of carriers therethrough. As a result, the laser diode of FIG. 1 suffers from the problem of high resistance.
In order to eliminate the foregoing shortcomings of the conventional planar laser diode of FIG. 1, an inventor of the present invention has previously proposed a planar laser diode shown in FIG. 3, wherein the laser diode is constructed on a semiconductor substrate 31 of the first conductivity type. On the upper major surface of the substrate 31, there are provided a first reflecting structure 32 formed of an alternate stacking of a semiconductor layer 32.sub.1 and a semiconductor layer 32.sub.2 both doped to the first conductivity type, and a first cladding layer 33.sub.1 of the first conductivity type is provided on the first reflecting structure 32. On the first cladding layer 33.sub.1, an undoped first barrier layer 33.sub.2 is provided and an undoped active layer 33.sub.3 is provided on the first barrier layer 332, wherein the active layer 33.sub.3 is sandwiched by the foregoing barrier layer 22.sub.2 from below and by an undoped second barrier layer 33.sub.4 from above to form a quantum well layer characterized by a quantum level of carriers. On the second barrier layer 33.sub.4, there is provided a second cladding layer 33.sub.5 of the second conductivity type, wherein the layers 33.sub.1 -33.sub.5 form an optical cavity 33. Further, a current confinement structure 34 of the first conductivity type is provided on the optical cavity 33 except for the part of the optical cavity 33 wherein the laser oscillation takes place, and a contact layer 35 of the second conductivity type is provided on the current confinement structure 34 such that the contact layer 35 establishes an intimate contact with the exposed surface of the second cladding layer 33.sub.5. Thus, a p-n junction is formed at the interface between the current confinement structure 34 and the contact layer 35, wherein the depletion region associated with the p-n junction pinches the path of the injection current such that the current flows selectively where the contact layer 35 establishes an intimate contact with the cladding layer 33.sub.5.
On the upper major surface of the contact layer 35, there is provided a second reflecting structure 36 formed of an alternate stacking of undoped semiconductor layers, wherein the second reflecting structure 36 is provided selectively in correspondence to the path of the optical beam in the laser diode, and an upper electrode 37 is provided on the upper major surface of the contact layer so as to surround the second reflecting structure 36. Further, the laser diode of FIG. 3 includes an anti-reflecting coating 38 provided on the lower major surface of the substrate 31 in correspondence to the part where the optical beam passes through, and an electrode 39 is provided on the lower major surface of the substrate 31 so as to surround the anti-reflecting coating 38. Thus, the optical beam produced by the laser oscillation in the laser diode of FIG. 3 is emitted through the anti-reflecting coating 38 in the downward direction from the substrate 31.
In the planar laser diode of FIG. 3, it should be noted that the reflecting structure 36 at the upper major surface of the contact layer 35 does not form a part of the current path. Thus, there is no need for doping the semiconductor layers that form the reflecting structure 36, and the optical absorption caused by the reflecting structure 36 is substantially reduced. As the injection current does not flow through the reflecting structure 36, the problem of the potential barrier associated with heterojunction interface in the reflecting structure 36 preventing the flow of carriers is successfully eliminated.
In the structure of FIG. 3, however, there remains the problem of optical absorption that is caused by the contact layer 35. It should be noted that the contact layer 35 is doped to a high impurity concentration level in order to provide a current path for the injection current, such that the current injected from the electrode 37 reaches the active layer 33.sub.3 through the contact layer 35 with a minimum resistance. As the optical beam is reflected back and forth between the reflecting structures 32 and 36 in the structure of FIG. 3, there inevitably occurs a substantial absorption of the optical beam in the contact layer 35.